The measurement of refraction, or visual ametropia (blurring and/or astigmatism of the eye), is a requisite step before correcting the same. The quality of vision is of utmost importance for the well-being of a person, because it is estimated that human beings receive about 80% of the information of their surrounding by the sense of vision.
A large percentage of the population suffers some ametropia, and in some cases there exists a notable variation in incidences depending on the geographic region. For example, it is a known fact that among Asiatic populations there is up to an 85% incident rate for myopia in certain regions of the continent. It also occurs that the eye undergoes a series of changes due to age related developments, which result in a loss of the ability to correctly focus on objects located at different distances. What is known as presbyopia or tired eyes affects 100% of subjects over 50 years old.
In light of the above the conclusion may be drawn that the measurement of visual defects caused by refractive ametropia is a field meriting worldwide interest with a potential market that embraces 100% of the population.
An issue that is inseparably linked to the measurement of ocular refraction is the available visual corrections. The characteristics of these determine the requisite quality and precision for measuring visual quality.
A quick historical review of visual corrections starts with the first ophthalmic lenses that corrected blurring, and which began to be used more extensively beginning in the thirteenth century, especially for compensating presbyopia or tired eyes that appears in the human eye at about the age of 50 and onward. Astigmatism was not adequately measured and corrected with cylindrical lenses until the nineteenth century, all indications being that the pioneer in the method was the renowned scientist Thomas Young. Since then the advances made to ophthalmic lenses have been modest. Nowadays, the large majority of subjects who use eyeglasses or contact lenses, which made their appearance around the mid twentieth century, only correct their blurring and/or astigmatism.
It was not until well into the twentieth century that the first methods of subjective measurement of the optical quality of the human eye appeared, clearing revealing the existence of other optical defects or aberrations that compromise the quality of vision, besides the already known blurring and astigmatism.
Among the numerous existing methods and techniques, the Hartmann-Shack sensor may be mentioned due to the large acceptance and widespread use it currently has. The first reference to its use on the human eye appears in the works of Liang, B. Grimm, S. Goelz, and J. F. Bille, “Objective measurement of WA's of the human eye with the use of a Hartmann-Shack wave-front sensor,” J. Opt. Am. A 11, 1949-1957 (1994); J. Liang and D. R. Williams, “Aberrations and retinal image quality of the normal human eye,” J. Opt. Soc. Am. A 14, 2873-2883 (1997); and also P. M. Prieto, F. Vargas-Martin, S. Goelz, P. Artal, “Analysis of the performance of the Hartmann-Shack sensor in the human eye”, J. Opt. Soc. Am. A, 17, 1388-1398 (2000). Nowadays there are commercial versions that implement this method with much success in certain applications.
The possibility of objectively measuring the aberrations favoured the appearance of adaptive optics applied to the human eye at the start of the twenty first century. By means of this technique the optical aberrations can be corrected in a precise manner and in real time following their measurements. This is obtained by means of the use of phase modulators which can be based on the use of liquid crystal or deformable mirrors, in all of their variants and modalities. A pioneering work in this field was published by E. J. Fernández, I. Iglesias, and P. Artal, “Closed-loop adaptive optics in the human eye”, Opt. Lett, 26, 746-748 (2001). This technique is the most immediate predecessor to the so called visual simulators. These are instruments that enable the measurement of the optical quality of the eye, and its manipulation by means of aberration generating devices. To date it use has been restricted to the field of scientific research and, preferably, to monocular cases. An early work in this field was described in E. J. Fernández, S. Manzanera, P. Piers, P. Artal, “Adaptive optics visual simulator”, J. Refrac. Surgery, 18, 634-638 (2002).
The measurement of higher level optical aberrations, beyond blurring and astigmatism, has opened the door to its possible correction by means of ophthalmic elements, such as lenses, contact lenses, intra-ocular lenses that are surgically implanted in the eye of the patient, or refractive cornea surgery, in which different profiles can be sculptured on the cornea of the subject for its refractive correction.
However, it is now known that the refraction does not provide an objective measurement of the optical quality of the eye in an absolute sense. Although optical quality is strongly linked to the quality of vision, there are no methods that are capable of estimating the sharpness or sensibility to contrast of a subject based on the values of the different optical parameters that characterize the eyes. Recent works have clearly demonstrated this limitation, as shown in the article of P. Artal, L. Chen, E. J. Fernández, B. Singer, S. Manzanera, D. R. Williams, “Neural compensation for the eye's optical aberrations”, J. Vis., 4, 281-287 (2004). This is easy to understand when the phenomenon of vision is undertaken in an integral manner. Thus, the formation of images in the retina only constitutes the first step of a very complex process which involves the transduction of light into physical-chemical signals that are sent to the brain, and a subsequent psychological interpretation of them which ultimately produces the visual sensation or perception. In this manner, it is accepted that the measurement of refraction has a strong subjective component that bears on the treatment of the patient, who ultimately must decide which correction gives him the best visual perception.
In this context we find that, despite the enormous advances of the last few years in the measurement of optical quality of the eye, traditional phoropters, those based mainly on conducting simple visual tests by means of lenses with different graduations until the subject perceives the best image, continue to be the most used throughout the world.
These phoropters could be grouped together as a first classification of those which are placed in front of the eyes of the subject, like some eyeglasses. These incorporate a series of purely mechanical controls that enable rotating astigmatic lenses to position them correctly, as well as the interchange of spherical lenses for the correction of simple blurring, and which are currently the most utilized type of phoropters in clinical practice throughout the world. Other alternatives, essentially variants of the same concept, enable the introduction of colour filters, polarizing filters, etc. In all of them the visual tests or stimuli are projected before the subject on a screen or similar, in an independent manner with respect to the phoropter per se.
Within this family of instruments for the measurement of ocular refraction we find numerous patent documents, such as document US 2 003 009 063 A1, which introduces improvements for the correct control of the variables that could be adjusted in the phoropter by the examiner in very low lighting conditions.
Document U.S. Pat. No. 7,156,517 B2 shows various improvements which above all affect the ergonomics of the examiner, allowing him to be more comfortable during the control of the lenses that he is introducing into the phoropter in order to measure the ocular refraction. This is obtained by means of new lighting systems. In any event, this is an invention that does not disclose direct improvements for the patient or subject who is undergoing the refraction tests.
Document U.S. Pat. No. 5,812,241 A discloses a much more compact phoropter in which the spherical and astigmatic lenses are directly incorporated into a type of interchangeable roulette, organized in such a way that it reduces the size of the instrument. Again, as in the previous document, this is an invention that provides a modest benefit to subject who is undergoing the refraction tests. In any event, the proposed instrument follows along the same lines as this family of phoropters.
The same idea for improving the interchange of ophthalmic lenses by means of a reel that holds them, and in particular, disclosing a method that makes its control more efficient, is described in document JP 8 182 649 A, for a phoropter with two rotational channels by means of a mount adapted specifically for this use.
The invention disclosed in document U.S. Pat. No. 4,861,156 introduces a control unit for visual stimuli that are shown to the subject during the use of the phoropter. Basically this enables controlling the visual tests with the phoropter itself, avoiding the need of the examiner to change positions during the process, thus improving his comfort.
The phoropter described in document U.S. Pat. No. 5,223,864 introduces some sensors in the lenses themselves, which make it possible to find, for example, the angle that an astigmatic lens is using, within the scheme of the classic phoropter described in all of the previous documents of this type. Consequently, this is an invention that improves the manipulation of the instrument on the part of the examiner.
More recently the first phoropters controlled electronically by means of special lenses have made their appearance, the strength of which depends on the electrical signal sent. The concept here is different from that of the family of phoropters presented above, in which the trial lenses are mechanically interchanges during the refraction process. This is done by substituting the lenses either by rotation or by switching them. With electro-activated phoropters that make possible the practical implementations described in documents U.S. Pat. Nos. 7,264,354 B2 and 7,533,993 B2, the examiner can adjust the correction applied to each patient during the process in a digital manner, which, consequently, is much more precise than previous methods. Furthermore, the smallest step or resolution which refraction can obtain now depends on the minimal electrical signal that can be sent to control the strength of the lens. In these instruments the measurement of astigmatism is not obtained by means of variable strength lenses, and so for this ametropia it is still necessary to recur to the previous methods of mechanically interchanging and rotating the lenses.
Document U.S. Pat. No. 4,943,162 discloses an invention that facilitates the use of lenses with astigmatism in the context of a phoropter. The invention proposes a method and instrument that implements it for the rotation of two series of astigmatic lenses in a systematic manner to determine the refraction of the subject.
In current state of the art there is clearly an enormous breach between modern means of correction of ametropic refractions, and of optical aberrations in general, with the measurement of the refraction or the subjective quality of vision. Thus, nowadays adequate technology exists for manufacturing ophthalmic lenses and contact lenses with phase profiles that go beyond blurring and astigmatism. Intraocular lenses with aspheric profiles are already being mass produced, including the diffractive type for surgical implantation. Along the same lines, modern surgical techniques in refractive surgery, by means of the latest generation of lasers with sophisticated guidance systems, enable trimming the cornea of the patients with high precision, thus opening the door to high order correction of optical aberrations.
However, as has previously been made manifest, the capacity and operation of current phoropters is quite removed from providing the necessary features for evaluating the vision of the patents, on the one side in a entirely digital manner, and on the other by incorporating the possibility of seeing through the phase profiles, or correction, beyond blurring and astigmatism, which can limit in a substantial manner the development of new corrections systems.